The present invention relates to collision detection and bit arbitration in a digital data system and more particularly to a novel method and apparatus for threshold-less collision detection and bit arbitration in a chirped-frequency-shift-keyed (CFSK) data system.
Channel sensing has often been suggested as a method for improving the efficiency of digital transmission systems of the contention type, such as those using ALOHA random access protocols and the like. In such systems, transmission is started on a data channel only when that channel is sensed to be idle. For example, in a Carrier Sense Multiple Access/Collision Detection (CSMA/CD)-type protocol system, system efficiency can be even further increased by detecting actual collisions, i.e. simultaneous use of a channel by more than one data-transmitting entity, and causing the cessation of all colliding transmissions as soon as such collisions are detected. Further improvement is possible by biasing collision decisions in favor of one logic state (either the logic 1 or the logic 0 state) when collisions are detected between complimentary bit state transmissions. In such a protocol, a transmitting data source which had sent the "favored" data state continues its transmission while other colliding transmitters are caused to cease operation. Data receivers are, in turn, caused to decide that the "favored" bit was received whenever such a collision is detected. This form of protocol may be referred to as "collision detection with bit arbitration" (CDBA).
A major requirement for a synchronous CDBA protocol is the ability for a data station to detect collisions between complimentary bit transmissions in any given bit time interval. Once a collision is detected, bit arbitration follows. In such systems, collisions between transmitting data stations in the identical logic-state bits do not induce bit errors at a receiving station and is therefore not a problem. While many forms of data transmission systems may utilize such a synchronous CDBA scheme, it has hitherto not been implementable in a data transmission system in which a bit of data is modulated onto a radio-frequency carrier (typically in the frequency range between about 10 KHz. and about 1 MHz.) and wherein a synchronous chirp frequency-shift-keyed (CFSK) data modulation technique is utilized. CFSK methods and apparatus are described and claimed in co-pending application Ser. No. 301,706 assigned to the assignee of the present application and incorporated herein in its entirety by reference. It is highly desirable herein in its entirety by reference. It is highly desirable to provide a method for collision detection and bit arbitration in a multiple access data communication system using CFSK modulation for transmission of the data logic bit state. Hitherto, collision detection in RF modulation schemes has been implemented by comparing the detected output of a pair of matched filters, with one filter being utilized for detecting the logic 1 bit state and the other filter utilized for detecting the logic 0 bit state. The matched filters in the logic 1 and logic 0 signal paths may be provided with either a preset or adaptive threshold, such that a collision is detected if the output of both signal path matched filters simultaneously exceeds the threshold. While this approach has found some degree of success, it encounters difficulty if system noise is relatively high or if the data channel attenuation is subject to rapid fluctuation, causing choice of a threshold to become relatively difficult. A technique for collision detection and bit arbitration which does not require a threshold is therefore highly desirable.